Overloading :+= in Ruby

Question

I want to define a custom :+= method on my Ruby class that modifies the instance. However, the following code raises syntax error, unexpected '=', expecting ';' or '\n'

def +=(other)
  @value += other.to_f
end

What are my options here? I see that Fixnum has :+@ and :-@ methods, but I don't exactly see what they do in the documentation. Are those the methods I want to write?

Answer 1

Unlike C++ or other languages with robust overrides, there's no such method as += in Ruby.

What's happening internally is a sort of expansion. For example:

x += n

If x is a variable, then this is equivalent to:

x = x.send(:+, n)

If x= is defined, then this is equivalent to:

send(:x=, x.send(:+, n))

Whatever you need to do to override must be to redefine x= or + on the class of x.

Remember that the + method should not modify x. It's supposed to return a copy of x with the modification applied. It should also return an object of the same class of x for consistency.

Your method should look like:

def +(other)
  # Create a new object that's identical in class, passing in any arguments
  # to the constructor to facilitate this.
  result = self.class.new(...)

  result.value += other.to+f

  result
end

Answer 2

I am not really sure, but you do not need the = operator.

def +(other)
  @value += other.to_f
end

Example from here:

class Money
  attr_accessor :value

  def +(b)
    result = dup
    result.value += b.value
    return result
  end

  def -(b)
    result = dup
    result.value += b.value
    return result
  end
end